Normal Maki is a serene, quiet girl who takes simple pleasures such as painting or just being with her friends, and in her youth her poor health made her profoundly unhappy, as it denied her most of these simple delights; as a result of Takahisa Kandori's experimentation, her whole self was splintered into several personality fragments. As a result of the events of Megami Ibunroku Persona, her condition was stabilized and she was reunited to completeness, though she was left with the memories of the horrors she had unwittingly caused.

This became a major motivation at the time of Persona 2, leading her to elect the career of a therapist so she could help others to make up for the destruction she had once wrought.

"A chronically ill classmate who's been hospitalized for over a year now."

—PSP introduction

Maki Sonomura is a frail girl who has been sick all of her life. When she's not in too much pain, she often paints disturbing still-life pictures, escapes into her "Ideal" world, and waits for her friends to come visit her.

Maki's thoughts somehow came into contact with the DEVA System, and Takahisa Kandori takes advantage of this by kidnapping Maki from the hospital, and uses her unconscious mind with the DEVA System to create a parallel world that is a reflection of Maki's "ideal" version of Mikage-cho. In this world, Maki is healthy, demons wreak havoc everywhere, the hospital and police station do not exist, and the city has two sides, one light and one dark, divided in half by an ominous barrier.

Part of Maki accepts, and at the same time rejects this development. Thus, three manifestations of her psyche actually emerge: Maki, who joins the party, representing her "Ideal" self, and Mai and Aki, two alternate selves that represent her innocence and malevolence, respectively. The "Ideal" Maki is the same age, while Mai and Aki appear to look like Maki back when she was a child. Throughout the game, the Ideal Maki is the one helping the party, while the Real Maki is held captive by Kandori.

Upon knowing her true origin, the Ideal Maki escapes into her own self-pity and refused to accept the truth, until the hero convinces her to snap out of it, and gets the Green Compact from Maki.

The party also finds a Chaos Mirror Shard from Kandori's mirror, and a Broken Compact. The two become a Red Compact, and the party saves the Ideal Maki with the help of Setsuko Sonomura and Mai. Mai tells them that there is one more Compact, held by Maki's true self in the Sea of Souls beneath Alaya Cavern. Upon reaching the bottom of the cavern, the Ideal Maki convinces the Real Maki to give up the Blue Compact, and she asks them to go as she doesn't want the hero to see her as she is now.

Back at the school, the door in the library opens with the three Compacts, and the party enters the Avidya World. They find Pandora, an empty form of Maki who devoured Aki and stole the core of the DEVA System from Kandori while serving him. She plans to use it to create a world of nothingness, but the party defeats her and Ideal Maki convinces her to merge back together. Once Pandora is defeated, Ideal Maki reveals that since she's just a shadow created from the real Maki's heart, she cannot go back to the real world with them. As the rest of the party returns to the real world, Ideal Maki kisses the protagonist and confesses her feelings to him before he returns as well.

A few months later, the real Maki attends the graduation ceremony with everyone else. Afterwards, she and the party, along with Chisato Kasai and Yosuke Naito, plan to go to the theme park Dreamland. As the party leaves, Maki stops off at the school, looking at the photo of her with her classmates before taking the (normal colored) compact Pandora left behind.

In the epilogue, it's revealed that Maki has been completely cured of her illness and matured into a cheerful, positive woman. She won an award at a major art exhibit and has received lucrative commission offers, but her priorities still lie on spending her time with her mother. She was recently seen enjoying dinner with her mother and a man who seemed to be her boyfriend.

In Revelations: Persona, her name is changed to Mary, while Mai and Aki's names were changed to Mae and Maggie. Her hair is also recolored to a lighter brown.

Maso: Maki's Initial Persona. Maso's awakening was never shown so it's assumed it took place between the time Maki, Elly and Naoya left the school and arrived at the Abandoned Factory, since Maki never mentioned being a Persona-user and nobody (including Maki herself) showed any surprise at Maso's first on-screen appearance.

Tensen Nyannyan: Awakened after the fight against the Harem Queen/Chisato Kasai. Tensen Nyannyan is a very useful Persona due to its Samarecarm spell which was used multiple times for healing, like saving Chisato's life after she attempted suicide by slicing her own throat with a glass shard after being defeated, for example. Tensen Nyannyan also dealt the killing blow on Hypnos using her Heat Wave attack.

"Yukino's friend who's studying as a counselor. She seems to have some guilt in her past. She awakened as a Persona-user in high school."

—Her introduction, Persona 2: Innocent Sin

Maki now works as a psychiatrist's assistant in Hiiragi Therapy, where she is found and recognized by her old classmate Yukino Mayuzumi. She feels that helping people is the best way for her to cope with the fact that she almost destroyed the city, due to the fact that her imaginary world almost superimposed the real one.

Later in the game, if the correct rumor is spread, she will receive a Legendary Flower from the Time Count which she gives to Jun Kurosu as his ultimate weapon, mentioning what the count said about it being "a representation of destiny that must be passed on."

Maki still works in Hiiragi Therapy. When she meets Maya Amano, she can immediately tell from the resonance that Maya has a Persona and that she is acquainted with Yukino Mayuzumi. She later appears at the sewer with Reiji Kido in Kei Nanjo's route, assisting the party by coordinating their movements with the floodgate controls. She will later accompany the rest of her former classmates in battle once again, this time to repel the New World Order soldiers that are trying to occupy Sumaru City.

Maki briefly appears in the flashbacks of Kei Nanjo and Eriko Kirishima during their last day in St. Hermelin High. In Eriko's flashback, she encouraged Eriko to confess her love for the Boy with Earring but Eriko failed due to shyness.

In Persona 3, a bow called "Mary's Resolve" can be found in Tartarus' Monad Block. This is connected to Maki's Revelations: Persona name, Mary. In Persona 3 Portable, this is changed to "Maki's Resolve," further proving the connection.